1. Field of the Invention
The present invention relates to an apparatus for recording/reproducing optical data and a method for the same and, more particularly, to an apparatus and a method capable of performing accurate focus-servo control.
2. Description of the Related Art
Generally, optical disks are used in optical data recording/reproducing apparatus to record data optically. Each optical disk has on its surface a spiral track or concentric tracks. A number of tiny pits are cut in each track. The length of the pits varies in accordance with the kinds of data items the pits represent. The surface of the optical disk is divided into a number of sectors. Addresses are assigned to these sectors. The optical disk comprises a circular substrate made of glass or plastics, and a data-recording layer made of metal such as tellurium or bismuth.
Microscopically speaking, the recording surface of the optical disk is not perfectly flat. It warps by about .+-.300 .mu.m. While the disk is spinning, its surface moves up and down repeatedly. This phenomenon is known as "surface vibration." To record data on, and reproduce it from, the optical disk with high accuracy, a laser beam must be focused exactly on the surface of the optical disk. Due to the surface vibration, the laser beam cannot be correctly focused onto the surface of the disk. In order to compensate for this focus error, an objective lens, through which the laser beam is applied to the disk, is moved in the axial direction of the beam. This operation is called focusservo control.
Focus-servo control techniques are disclosed in "Optical Memory and Optical Magnetic Memory Art Collections", issued by K. K. Science Forum, Section 3, 113-120 (Oct. 1983).
Typically, an apparatus, which performs the focus-servo control, comprises a focus-error detector, an amplifier, and an actuator. The focus-error detector is designed to detect the distance between the surface of an optical disk and the focal point of a laser beam. the amplifier amplifies the signal generated by the detector and representing the said distance. The actuator drives an objective lens in the axial direction of the laser beam in accordance with the signal amplified by the amplifier. The focus-error detector, the amplifier, and the actuator constitute a servo loop. Several methods of detecting the focus error are known. One of these methods is called "knife edge method," which will be explained.
The knife edge method uses a shield which is located at a position where the beam reflected from the optical disk has the smallest diameter, while the focal point is located on the surface of the optical disk. The shield is as thin as the edge of a knife. This is why the method is called "knife edge method." When the laser beam is focused correctly on the surface of the optical disk, the shield does not shield the beam reflected from the disk, and the beam is applied equally to the respective inputs of focus-error detectors, for example, a pair of photosensors, which are arranged besides the shield. Hence, the electric signals output from the photosensors are at the same level. When the distance between the objective lens and the surface of the disk becomes shorter due to the surface vibration of the disk, the shield blocks part of the beam spot, whereby the beam is applied unevenly to the respective inputs of the photosensors. As a result, the electric signals output from the photosensors are at different levels. These electric signals are amplified by an amplifier and input to a subtraction circuit such as a differential amplifier. The subtraction circuit outputs a focus-error signal. The focus-error signal is input to an actuator. The actuator moves the objective lens by a distance in accordance with the amplitude of this signal, so that the laser beam is focused correctly on the surface of the optical disk. In which direction the actuator moves the objective lens is determined in accordance with whether the disk is located nearer to, or farther from, the objective lens than required. More precisely, when the disk surface is nearer to the lens than necessary, the actuator moves the lens away from the disk; when the disk surfaces is farther from the disk surface than required, the actuator moves the lens toward the disk. Since the actuator moves the objective lens this way in accordance with the focus-error signal, the laser beam is correctly focused upon the surface of the optical disk.
The focus-servo control method described above has, however, a drawback in that the servo loop does not have a sufficiently good response. To make matters worse, the gain of the servo loop is liable to change. Therefore, the servo loop cannot function to correct a focus error resulting from a relatively large surface vibration of the optical disk.